This invention relates to the distillation of methane and optionally lower-boiling components from a methane-containing crude gas, e.g., natural gas.
Such a method is described in DAS [German Published Application] No. 1,619,728 in two variations, the contents of said DAS being incorporated by reference herein. Both cases involve the separation of a methane-ethylene mixture.
The first of these conventional processes resides in using a single auxiliary cycle for condensing overhead vapor to provide reflux liquid at the head of the column and for providing heat to produce reboiler vapor in the column sump; the medium of this auxiliary cycle contains methane. However, this process has the disadvantage that the column pressure can not be more than 3 bar, and the pressure difference to be provided by the cycle compressor must be at least 30 bar. The compression ratio required in the auxiliary cycle is thus at least 11.
In the second conventional process variation, a mixture cycle is utilized for cooling the head of the column to provide reflux liquid and for heating the column sump to provide vapor; the medium of this mixture cycle contains at least two components, expecially methane and ethylene. The cycle is subdivided into two branches after the partial condensation of the cycle medium resulting from the heating of the column sump. Thereafter only the lower-boiling component is used, after throttle expansion, for the cooling of the column head. The permissible column pressures according to this mode of operation range between 2.5 and 5 atmospheres absolute. The cycle medium must be compressed to maximally 18 atm. abs., there being the prerequisite that the cycle pressure during heat exchange with the head product of the column is to be lower than the column pressure, since the heat exchanging media of the head cooling on both sides involve greatly enriched methane. The compression ratio to be produced by the cycle compressor is, in this case, at least about 4, and in the mode of operation disclosed in DAS No. 1,619,728 even 15-18.
The disadvantages of the two conventional variations of the process reside, accordingly, in the relatively low column pressures involved and the relatively high compression ratios required in the auxiliary cycle. As a consequence, there is the necessity of providing large dimensions for the column and of incurring a high operating energy consumption when conducting the process.